A still from the video sent by Cho Sueng-hui to NBC in between his two killing sprees. Photograph: NBC/AP

The Virginia campus killer paused in the two-hour gap between the first and second shootings to post a package containing a video, photographs and a multi-page statement to a US television headquarters.

NBC News received the package yesterday and handed it to the FBI. It included a digital photograph of Cho Seung-hui, 23, with his arms spread wide, a handgun in each. He was wearing a black baseball cap backwards, black gloves, and a vest with an array of pouches - the same clothes described by witnesses to the shooting.

In the video, almost all of it a rambling rant, he said: "I did it ... I had to do it." Much of it is directed against the wealthy, saying he is speaking on behalf of the weak and the humiliated. He referred to figures ranging from Jesus to the Columbine high school killers.

"You forced me into a corner and gave me only one option," Cho said.

Police and university officials earlier disclosed that Cho had been referred to a psychiatric institute for a short stay two years ago after fears he might be suicidal.

The package was stamp-marked 9.01am on Monday. The first shooting was about 7.00am and the second spree began soon after the package was posted. The numbering of the address was slightly wrong, delaying arrival by a day.

The existence of the package was disclosed at a press conference at the campus by Steve Flaherty, the head of Virginia state police, who said: "This may be a very new, critical component of this investigation. We're in the process right now of attempting to analyse and evaluate its worth."

The video was broadcast last night. NBC News's president, Steve Capus, said it contained material that was "disturbing, very disturbing - very angry, profanity-laced". He said it did not include any images of the shootings but did include "vague references", including "things like 'This didn't have to happen.'" The package came after a similar note railing against rich kids was found in his room at Virginia Tech, which is one of the most prestigious engineering universities in the US.

Thirty-three people, including Cho, were killed in the shootings on Monday. A further eight were still in hospital last night. The police said identification had not yet been completed so the authorities were unable to release the bodies.

Earlier, the governor of Virginia, Tim Kaine, announced an independent investigation into the slow response of the university and police. Mr Kaine, who addressed a memorial service at the campus on Tuesday, warned against snap judgments. He said he had "nothing but loathing for those who take the tragedy and make it political". The inquiry, led by Gerald Massengill, a former head of Virginia state police, will focus on the two-hour gap between the first shooting, which claimed two lives, and the second, which claimed 31. It will also examine whether the university should have been monitoring Cho more closely.

Few involved have so far called for the inquiry to look into the state's relaxed gun laws, in particular why Cho was allowed to buy handguns, given he had been in a psychiatric institution. The review is being set up at the request of the university's president, Charles Steger, who has taken the brunt of the criticism so far.

The police removed two computers from Cho's room, a digital camera, notebooks, and a chain and lock.

The police said yesterday they had completed forensic work at the hall, where Cho shot 30 staff and students before shooting himself. It will remain closed for the remainder of the academic session.

One of the many unanswered questions about Cho is why, after killing Emily Hilscher and Ryan Clark in their dormitory, he chose Norris hall for his shooting spree.

One reason why police failed to close down the university was because they initially assumed the two dormitory killings were a "domestic" incident and began searching for Ms Hilscher's boyfriend, Karl Thornhill. They were questioning him when news came through of the shootings at Norris hall.

There was continuing nervousness on the campus yesterday. Police received a threat on the life of Mr Steger in the morning and then heard a report of a suspicious person in the building where his office is housed. Police rushed to the building but it was a false alarm.

In a garden opposite the president's office, students, staff and family have been laying flowers and set up panels on which to write messages of remembrance.

Meanwhile the Korean Yonhap news agency said yesterday that Cho's parents, who live in Centreville, Virginia, had been taken to hospital suffering from shock rather than from a suicide attempt, as other Korean media had been reporting.

Excerpts from the video message that Cho sent to NBC

"You had a hundred billion chances and ways to have avoided today ... Now you have blood on your hands that will never wash off."